This invention relates to low-density, relatively ductile aluminides exhibiting a cubic L1.sub.2 crystal structure. Specific exemplary aluminides include ternary Al-Ti-M systems (wherein M is Cu, Ni, Co, or Fe). Further, quaternary systems in which V, Nb, or Ta are added to the ternary elements of the Al-Ti-M system have also been found to exhibit a Ll.sub.2 structure and desirable properties.
Aluminum rich intermetallic alloys such as TiAl.sub.3 have long been known to have potential use as low-density elevated-temperature materials. For example, many ordered alloys retain high strength and high modulus at elevated temperatures. In addition, they tend to demonstrate higher oxidation resistance than conventional alloys. However, such alloys have been characterized by extreme brittleness. This brittleness, or low ductility at ambient temperatures, is objectionable for structural applications, and results from diverse reasons such as insufficient numbers of slip systems, limited cross slip, locking of dislocations by impurities, and weak intergranular bonding. Past work has circumvented these problems by using ordered alloys only as second phase particles added to strengthen a disordered matrix, as illustrated by various nickel-based superalloys.
As discussed herein, the term "ordered alloys" will refer to alloys having two or more atomic species which occupy specific sites in the crystal lattice. Ordered alloys have been extensively studied since the mid 1950s, with little success achieved in overcoming ambient temperature ductility deficiencies. Aluminides, based upon titanium, iron, and nickel, were identified as being among the more interesting systems in terms of structural properties. Due to inability to overcome basic problems of brittleness and lack of ductility, various alternative uses for such materials were developed, such as the use of nickel and cobalt aluminides for coating turbine hardware, or the use of iron-cobalt alloys in transformers in view of their high magnetic permeability. Ordered alloys such as Ni.sub.3 Al and Ni.sub.3 Nb were used as strengthening phases in steels. The potential for use of ordered alloys in structural applications increased with ductility improvements achieved in TiAl and Ti.sub.3 Al based alloys produced through powder metallurgical and alloying techniques. Rapid solidification techniques led to renewed interest in iron and nickel aluminides. One example is known to have been published in the last decade, where hexagonal Co.sub.3 V was transformed by alloying to the cubic L1.sub.2 structure through control of electron-to-atom concentrations. However, this is the only system in which such a transformtion is known to have been successfully made. The drawback of this material is that it is based on a high density intermetallic, and no such attempts are known to have been made in the lighter weight aluminide systems. For a more extensive discussion, attention is directed to "STRUCTURAL USES FOR DUCTILE ORDERED ALLOYS," a report of the Committee on Application Potential for Ductile Ordered Alloys, National Materials Advisory Board, National Academy Press, Washington, DC, 1984.
In U.S. Pat. No. 4,292,077, Blackburn et al. disclosed titanium-aluminum-niobium alloys having a compositional range in which ductility at low temperature is achieved. This reference relates specifically to the addition of from eleven to sixteen atomic percent niobium to binary ordered alloys of the Ti.sub.3 Al type. In this technique, it is not fully understood why the ductility improves, but it is known that the improvement is not the result of a change in the crystal structure. Such alloys may be stated in nominal weight percent as Ti--13/15 Al-19.5/3ONb. In one embodiment of the invention, vanadium partially displaces niobium, thereby lowering density, while favorable high temperature properties are retained.
In U.S. Pat. No. 4,294,615, Blackburn et al. teach alloys based upon TiAl gamma phase structure, to which binary structure up to four percent vanadium has been added. The TiAl gamma alloy system was selected as having the potential for being lighter, inasmuch as it has lower density due to the high concentration of aluminum. Blackburn et al. recognize the tetragonal arrangement of the atoms of TiAl, and the different alloying characteristics of such a system as compared to the hexagonal crystal structure of Ti.sub.3 Al. Patentees found that in titanium alloys comprising a rather narrow compositional range of aluminum, between 48-50 atomic percent, various elements could be added for altering properties beneficially. Alloys with lower aluminum concentration have higher strength but ductilities much less than 1.5 percent, while higher aluminum concentrations than the specified range gave lower strengths and lower ductilities. The addition of 0.1 to 4 percent by weight of vanadium improved room and moderate temperature ductility without adversely affecting high temperature strength. Both ternary and quaternary systems were investigated, with vanadium being the primary additive material. In the quaternary systems suggested by the Patentees, beta promoters such as molybdenum and tungsten, and alpha promoters such as bismuth and antimony, were evaluated. As discussed relative to the previous reference, none of the additives change the crystal structure of the parent intermetallic.
In the article, "PHASE EQUILIBRIA IN THE COPPER-TITANIUM-ALUMINUM SYSTEM," by Piero Verdis and Ulrich Zwicker, Z. Metalkde, Volume 62 (1971), No. 1, pp. 46-51, the existence of the Ll.sub.2 phase CuTi.sub.2 Al.sub.5 is noted. This paper was primarily concerned with phase identification and stability ranges, rather than the identification of useful structural materials. It is worth noting that in this work no attempt was made to prepare a single phase T.sub.3 material and measure its mechanical properties.
Similarly, the article "PHASE EQUILIBRIA IN THE TERNARY SYSTEMS Ti-Fe-O AND Ti-Al-Fe," by Angelika Seibold, at Z. Metalkde, 72 (10:712-719), 1981, teaches the existence of a ternary Ll.sub.2 phase corresponding to the approximate composition Ti.sub.8 Al.sub.22 Fe.sub.3. Again, this reference is primarily directed to identification of various phases within the system, and does not touch upon the issue of properties for structural applications, nor teach the preparation of a single phase material corresponding to the composition.
It is important to emphasize at this juncture that the existence of a cubic Ll.sub.2 phase does not guarantee ductility, as illustrated by the approximately equal split amongst ductile and brittle Ll.sub.2 intermetallic compounds observed by Vvedensky and Eberhart ("TOWARD A MICROSCOPIC BASIS FOR MECHANICAL BEHAVIOR," Philosophical Magazine Letters, 1987, vol. 55, no. 4, 157-161). Moreover, it has been shown by C. T. Liu that L1.sub.2 intermetallic compounds do not necessarily show advantageous high temperature properties (Liu, "HIGH TEMPERATURE ORDERED INTERMETALLIC ALLOYS," C. C. Koch, C. T. Liu, N. S. Stoloff, eds; Proceedings of the Materials Research Society Symposium, Nov. 26-28, 1984; Boston; vol. 39, p. 265). Accordingly, it would not be anticipated that the transformation from a tetragonal DO.sub.22 TiAl.sub.3 structure to a cubic Ll.sub.2 structure would necessarily result in enhanced ambient temperature ductility, while retaining advantageous high temperature properties. Absent an indication of such properties, one would not be led to utilize such materials in structural components.